Conventionally, metal foam is a commonly used material as a porous light-weight structure. Such metal foam is manufactured through a method (in the case of a close type) of generating air bubbles within metal in a liquid state or a semi-solid state, or a method (in the case of an open type) of casting using an open-type foamed resin, such as a sponge, as a mold. However, since the metal foam has relatively poor physical properties, such as strength and stiffness, and high production costs, the metal foam is not practically used except in specific fields, such as aerospace.
As a material substituting for the metal foam, there is an open-type light-weight structure having a periodic truss configuration. Such a structure has the truss configuration designed to have the optimal strength and stiffness through minute mathematical/dynamical calculation, thus having excellent mechanical properties. As a shape of the truss structure, an Octet truss in which regular tetrahedrons and regular octahedrons are combined is the most general (R. Buckminster Fuller, 1961, U.S. Pat. No. 2,986,241). Here, since respective elements of the truss form a regular triangle, such an Octet truss has excellent strength and stiffness. Recently, a Kagome truss modified from the Octet truss has been announced (S. Hyun, A. M. Karlsson, S. Torquato, A. G. Evans, 2003. Int. J. of Solids and Structures, Vol. 40, pp. 6989-6998).
With reference to FIG. 1, an Octet truss 101 and a Kagome truss 102 are two-dimensionally compared. Differently from a unit cell 101a of the Octet truss 101, a unit cell 102a of the Kagome truss 102 has a structure such that both a regular triangle and a regular hexagon are provided at each side.
FIGS. 2 and 3 respectively illustrate one layer of each of a three-dimensional Octet truss 201 and a three-dimensional Kagome truss 202. Through comparison between a unit cell 201a of the three-dimensional Octet truss 201 and a unit cell 202a of the three-dimensional Kagome truss 202, one of important characteristics of the three-dimensional Kagome truss 202 is that the three-dimensional Kagome truss 202 has an isotropic structure and thus mechanical properties and electrical properties of a structural material or other materials having the three-dimensional Kagome truss 202 are uniform regardless of direction.
As a manufacturing method of a truss-shaped porous light-weight structure, several methods, as described below, are known. The first method comprises making a mold has a truss structure formed of a resin and then manufacturing a porous light-weight structure by casting metal using the mold (S. Chiras, D. R. Mumm, N. Wicks, A. G. Evans, J. W. Hutchinson, K. Dharmasena, H. N. G. Wadley, S. Fichter, 2002, International Journal of Solids and Structures, Vol. 39, pp. 4093˜4115). The second method comprises forming a net by periodically perforating a thin metal plate, bending the net to form a truss intermediate layer and then attaching face plates to the upper and lower surface of the intermediate layer (D. J. Sypeck and H. N. G. Wadley, 2002, Advanced Engineering Materials, Vol. 4, pp. 759˜764). In this case, to manufacture a porous light-weight structure having multiple layers, such as two or more layers, mounting a truss intermediate layer formed by bending a net on the upper face plate and then attaching another face plate to the upper surface thereof. The third method comprises weaving wire meshes using wires in two directions perpendicular to each other, and then stacking and bonding the wire meshes (D. J. Sypeck and H. G. N. Wadley, 2001, J. Mater. Res., Vol. 16, pp. 890˜897).
The above first method involves a complicated manufacturing process and high costs and is capable of manufacturing a truss-shaped porous light-weight structure using only metal having excellent castability and thus has a narrow application range, and a product obtained through the first method tends to have many defects and low strength in terms of characteristics of a casting constitution. The second method causes large material loss during a process of perforating the thin metal plate and does not cause a problem in the case of a sandwich plate material having one layer of the truss, but in order to manufacture a structure having several layers, multiple layers of the trusses are stacked and bonded and thus the number of boning portions is excessively increased and thus the second method is disadvantageous in terms of bonding costs and strength.
Further, in the case of the third method, the manufactured truss does not have an ideal shape, such as a regular tetrahedron or a pyramid, and thus has low mechanical strength, and the truss is formed by stacking and bonding the wire meshes in the same manner as the second method and thus the number of bonding parts is excessively increased and the third method is disadvantageous in terms of bonding costs and strength.
FIG. 4 illustrates a structure manufactured using the above third method, i.e., a light-weight structure manufactured by stacking wire meshes. It is known that such a method may reduce manufacturing costs, but since wires in two directions are simply woven like weaving of a fiber, the structure does not have an ideal configuration having the optimal mechanical properties and electrical properties like the above-described three-dimensional Octet truss 201 and three-dimensional Kagome truss 202 and the number of parts to be bonded is excessively increased and the third method is disadvantageous in terms of bonding costs and strength.
A general fiber-reinforced composite material is manufactured in the shape of a two-dimensional thin lamina, and if a thick material is required, laminas are stacked.
However, in this case, the laminas may be separated from each other and thus strength of the manufactured material is lowered. Therefore, a method in which fibers are three-dimensionally woven from the beginning and are then combined with a matrix, such as a resin, metal, etc., is used.
FIG. 5 illustrates a fiber-woven shape of such a three-dimensional fiber-reinforced composite material. Instead of fibers, using a material having large stiffness, such as a metal wire, a porous light-weight structure may be manufactured through three-dimensional weaving, as shown in FIG. 5. However, the porous light-weight structure also does not have the ideal Octet and Kagome truss configuration, and thus has low mechanical strength and different physical properties according to direction. For this reason, the composite material manufactured of the three-dimensionally woven fibers has poor mechanical properties.
Considering the above problems, the inventors (2 persons including Ki-Ju Kang) of the present invention developed a three-dimensional porous light-weight structure which is formed in a regular shape similar to the ideal Kagome truss or Octet truss shape by crossing continuous wire groups in six directions having an azimuth angle of 60 or 120 degrees with respect to one another in a space, and a manufacturing method thereof, and the contents of the three-dimensional porous light-weight structure and the manufacturing method thereof are disclosed in Korean Patent Reg. No. 0708483.
Further, in order to more effectively manufacture a three-dimensional porous light-weight structure, the inventors proposed a three-dimensional porous light-weight structure woven by helical wires which is assembled by forming continuous wires into a helical shape and then inserting the helical wires while spinning the same, and a manufacturing method thereof, and the contents of the three-dimensional porous light-weight structure and the manufacturing method thereof are disclosed in Korean Patent Laid-open No. 2006-0130539.
The above-described three-dimensional porous light-weight structures disclosed in the Patents filed by the inventors of the present invention have several advantages, such as excellent mechanical properties and mass production at low costs through a continuous process, as compared to the conventional structures. However, if these three-dimensional porous light-weight structures are manufactured in a rectangular parallel piped shape, which is widely used, the shape of unit cells located at the corners is not perfect and thus the three-dimensional porous light-weight structures are disadvantageous in terms of appearance and mechanical strength, and increase in arrangement density of wires is limited due to interference among the wires. Accordingly, the inventors propose manufacturing methods of new three-dimensional porous light-weight structures which have different shapes from the Kagome truss while being manufactured by wires formed in a helical shape.